When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons. Common uses for well fluids include lubrication and cooling of drill bit cutting surfaces, transportation of “cuttings” (pieces of formation dislodged by the cutting action of the drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
Drill cuttings can originate from different geological strata, including clay, rock, limestone, sand, shale, underground salt mines, brine, water tables, and other formations encountered while drilling oil and gas wells. Cuttings originating from these varied formations can range in size from less than two microns to several hundred microns, including clays, silt, sand, and larger drill cuttings.
After formulating a drilling fluid with desired rheological properties, one challenge during the drilling process is maintaining the properties of the drilling fluid during recycle and reuse. For example, as mentioned above, the drilling fluids transport drilled solids to the surface for screening and disposal. Recycling drilled solids into the wellbore is undesirable, as this can result in smaller sizes of drilled solids which can accumulate in the drilling fluid, ultimately affecting the properties of the drilling fluid. If the solids content increases, additional drilling fluid (water, oil, etc.) and other chemicals must be added to maintain the drilling fluid at its desired density, viscosity, and other physical and chemical properties for the drilling fluid to satisfy the requirements for drilling a wellbore. The drilling fluid and drill cuttings returned to the surface are often separated to maintain drilling fluid weight, thus avoiding costly dilution. The separated solids are then discarded or disposed of in an environmentally accepted manner.
In addition to drill cuttings, the drilling fluid returned to the surface may also contain entrained gases. Gases that may be dissolved in or entrained by the drilling fluid may include methane, hydrogen sulfide, and many other gases released from the formation being drilled. Upon return of the drilling fluid to the surface, such gases must be effectively handled. For example, entrained gases may reduce pumping efficiency. Further, if released from the drilling fluid, methane poses a fire or explosion hazard, and hydrogen sulfide is poisonous in even minute concentrations.
Several types of separation equipment have been developed to efficiently separate dissolved gases and the varied sizes of the weighting materials and drill cuttings from the drilling fluid, including shakers (shale, rig, screen), degassers, screen separators, centrifuges, hydrocyclones, desilters, desanders, mud cleaners, mud conditioners, dryers, filtration units, settling beds, sand traps, and the like. Centrifuges and like equipment can speed up the separation process by taking advantage of both size and density differences in the mixture being separated.